The term micro hybrid vehicles is used to refer to motor vehicles having an internal combustion engine, a rather conventionally dimensioned electric starting motor, in particular what is referred to as a crankshaft starter motor (ISG, integrated starter generator), and a brake energy recuperation system for charging a starter accumulator with rather small dimensions. The internal combustion engine can be stopped, i.e. switched off, automatically by means of the automatic start/stop system if no propulsion is required, for example by coasting to a standstill, and can be re-started if the driver calls for power again. Alternatively, the internal combustion engine can easily be decoupled from the rest of the drive train and operated in the idling mode.
The automatic start/stop system stops the internal combustion engine on condition that the driver does not depress the accelerator pedal since at this moment no propulsion is required. The internal combustion engine is then decoupled from the rest of the drive train and either operated in the idling mode or, as already mentioned, switched off entirely. In both cases, there is a saving in fuel since the drag torque of the engine, which is obtained at the cost of the movement energy of the motor vehicle, is eliminated, with the result that the deceleration of the vehicle is reduced. In the first case, referred to as the coasting mode, in which the internal combustion engine carries on running in the idling mode, electric current is still generated, with the result that the power supply of the electrical loads in the motor vehicle is ensured in all cases. In the second case, referred to as free rolling or the rolling start/stop mode, the electrical loads in the motor vehicle have to be supplied with current by the battery, and the re-charging of the battery consumes additional fuel during times in which the internal combustion engine is running.
Both specified methods, the coasting mode and free rolling, can be applied in all drive trains which can be automatically opened in some way or other. This applies, of course, to automatic transmissions (conventional torque converter or double-clutch transmissions) and also to automated shifting mechanisms (ASM), continuously variable transmissions (CVT) and to manual shift transmissions which are equipped with an automatic, i.e. electrically or electronically activated, clutch.
The opening of the drive train at the beginning of a fuel saving phase influences the driving dynamics since the drag torque of the internal combustion engine which occurs when the accelerator pedal is not depressed disappears when the drive train is opened. In many driving situations, for example when traveling downhill, this may be undesired. It is also possible to provide that opening of the drive train occurs with a certain delay, for example in order to avoid continuous engagement and disengagement of the clutch. Various situations in which the automatic opening of the drive train can be prohibited or delayed with an automatic start/stop system are described in the following documents: EP 1 358 405 B1, EP 1 453 695 B1, EP 1 534 554 B1, DE 10 2008 029 453 A1, DE 10 349 445 A1, DE 10 221 701 A1 and DE 4 213 589 A1.